1. Field of the Invention
The present invention relates to a Differential Eight-value Phase Shift Keying (D8PSK) optical receiver, and particularly to a phase monitor to be used in a D8PSK optical receiver, and to a phase controlling method using the same.
2. Description of the Related Art
When the D8PSK method is employed, accurate setting of optical phase differences among arms of interferometers in a receiver is important. When the setting of the optical phase differences is not accurate, optical waveforms deteriorate beyond the tolerance. In order to obtain the accurate optical phase differences, a feedback control is performed generally. By the feedback control, phase adjustment signals are created for adjusting phases in a receiver by monitoring phase errors in the receiver such that the phase remains having the target value. In the dithering control which is a typical method of feedback, penalties are caused because the phase error is caused at the optimum point. And, the amount of penalties caused by the phase error in the D8PSK method is twice that in the DQPSK or more.
FIG. 1 shows the relationship between the phase shift of interferometers in a receiver and the Q-penalty in the cases of the D8PSK method and the DQPSK method respectively.
When the phase shift is zero, the Q-penalty is zero in both of the methods. However, with increased phase shift, the amount of the Q-penalties in the D8PSK increases twice faster than that in the DQPSK or more.
One example of control methods in the dithering control is explained here. In the dithering control, in order to determine whether or not the current phase is at the optimum point, the phase of the interferometers is slightly shifted. If the waveform deteriorates (if BER (Bit Error Rate) increases) with the above slightly shifted phase more than that with the original phase, it is determined that the original phase is closer to the optimum point. If the BER decreases with the slightly shifted phase, it is determined that the shifted phase is closer to the optimum point. The example of this is shown in the graph of the D8PSK in FIG. 1, in which even when a phase is at the optimum point (when phase is not shifted), the waveform deteriorates approximately by 0.2 dB due to the dithering control if the phase is shifted by 1 degrees by the dithering control being performed once.
Additionally, in the D8PSK, it is necessary to set to different phase states four interferometers for demodulation in order to demodulate all the signals.
Non-Patent Document 1 discloses a fundamental technique of the D8PSK. Non-Patent Document 2 discloses a technique in which the OSNR and the dispersion tolerance necessary for D8PSK systems are experimentally obtained. Non-Patent Document 3 discloses a technique in which parameters important to design of receivers and transmitters in the D8PSK are calculated.
Non-Patent Document 1
    Kim et al., “Direct-detection optical differential 8-level phase-shift keying (OD8PSK) for spectrally efficient transmission”, OPTICS EXPRESS Vol. 12, No. 15, 26 Jul. 2004Non-Patent Document 2    Serbay et al., “Experimental Investigation of RZ-8DPSK at 3×10.7 Gb/s”, 2005 IEEE WE3 9:00-9:15, p. 483-p. 484Non-Patent Document 3    Han et al, “Sensitivity Limits and Degradations in OD8PSK”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 3, MARCH 2005, p. 720-p. 722
Because optical communication systems employing the D8PSK method have been introduced just in recent years, many methods of controlling transmitters or receivers therefor are not suggested.